Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
In order to fuse toner onto a substrate it is beneficial to heat the toner to a point where the toner coalesces and become tacky. To an extent this causes the toner to flow into the fibers or pores of the substrate. Adding pressure increases the toner flow. Then, as the toner cools it becomes permanently attached to the substrate.
Because both heat and pressure are beneficial when fusing, most fusers include a heated element and a pressure-inducing element that form a nip. When a toner bearing substrate passes through that nip, heat from the heated element and pressure within the nip fusers the toner with the substrate.
One type of fuser uses a heated belt and a nip-forming backup (or pressure) roller. Reference, U.S. Pat. No. 5,450,182 to Wayman et al. on Sep. 12, 1995 and entitled "Apparatus and Method for Fusing Toner Images on Transparent Substrate," and U.S. Pat. No. 5,708,950 to Badesha et al. on Jan. 13, 1998 and entitled "Transfuser." Fuser belts have been heated by incorporating a resistive heating element on one side of an electrical insulating main substrate and by passing an electrical current lengthwise through the resistive heating layer. Furthermore, at least when using the fusing belt for transfusing (wherein the toner layer is transferred onto the belt and then fused from the belt onto a substrate), fusing belts have also included a release layer to assist toner transfer.
While resistively heated fusing belts have successfully been used for fusing, they generally suffer from limited life. This is at least partially due to the thermal effects of expansion of the belt. Since the main substrate is typically a thermally and electrically insulating material such as polyimide, sufficient mechanical strength and sufficient thermal flow were difficult to simultaneously achieve. Much stronger metallic fusing belts could not be used because they would short out the resistive heating layer. Despite their limited life, resistively heated fusing belts are desirable because they have the distinct advantage of rapid heating. This allows for energy conservation by enabling the fuser to be heated only when needed.
Therefore, a resistively heated fuser belt having a robust metallic substrate would be beneficial.